1. Field of the Invention:
The present invention relates to a long-life spark plug which is preferably employed in a gas heat pump or a cogeneration engine.
2. Related Art:
FIG. 9 shows a conventional spark plug 110 disclosed in Unexamined Japanese Patent Application No. 5-343159, published in 1993. The spark plug 110 comprises a central electrode 103 made of an electrically conductive member such as Ni alloy, copper, or copper alloy. A chip 105 is connected to a front (top) end 103a of the central electrode 103 by laser welding. The chip 105 is made of an electrically conductive material, such as Ir alloy or Pt alloy, having a fusing point higher than that of the central electrode 103. The chip 105, disclosed in this prior art, comprises a leg portion 151 inserted in a hole 1321 provided on the front end portion 103a of the central electrode 103. A larger-diameter portion 152 is formed integrally with the leg portion 151. The larger-diameter portion 152 has a diameter of 1.8 mm which is larger than that of the leg portion 151.
The front end portion 103a of the central electrode 103 and the larger-diameter portion 152 of the chip 105 are brought into contact with each other at their abutting faces. By performing the above-described laser welding, a fused portion 107 is formed at the abutting faces of the front end portion 103a of the central electrode 103 and the larger-diameter portion 152 of the chip 105. In an axial direction of the chip 105 (i.e., in the up-and-down direction of FIG. 9), the upper half of the fused portion 107 penetrates into the region of the chip 105. The lower half of the fused portion 107 penetrates into the region of he central electrode 103.
The above-described spark plug 110 has been preferably employed in a gas heat pump or a cogeneration engine. The life of the gas heat pump or the cogeneration engine is longer than the life of a conventional automotive engine. Accordingly, when the spark plug 110 is used for the gas heat pump or the cogeneration engine, the life of the spark plug 110 needs to be long enough.
The inventors of the present invention have performed an evaluation on the above-described conventional spark plug 110 through a test conducted under simulated operating environments of a gas engine. According to the test result the chip 105 was detached or disengaged from the central electrode 103 during a duration shorter than the life of the gas engine. A crack was recognized at the boundary surface between the larger-diameter portion 152 of the chip 105 and the fused portion 107. It is believed that the above-described detachment or disengagement of the chip 105 was caused as a result of an advancement of this crack.
Hereinafter, the causes of the above-described problem will be explained based on the experimental result and study conducted by the inventors.
First, the central electrode 103 is constituted by a member whose thermal expansion coefficient is larger than that of a member constituting the chip 105. For example, the central electrode 103 is made of nickel alloy having a thermal expansion coefficient of approximately 13.3.times.10.sup.-6 [deg.sup.- ]. The chip 105 is made of Ir alloy having a thermal expansion coefficient of approximately 6.8.times.10.sup.-6 [deg.sup.- ]. When the spark plug 110 is used practically, the spark plug 110 is subjected to repetitive heating and cooling cycles causing temperature variations of approximately 900.degree. C. Thus, a significant thermal stress is applied directly to or in the vicinity of the fused portion 107. Although the reason is not explicitly known, a bonding force between the fused portion 107 and the chip 105 is smaller than a bonding force between the fused portion 107 and the central electrode 103. Accordingly, the crack appears at the boundary surface between the fused portion 107 and the chip 105 due to the above-described thermal stress. And, the chip 105 removes or disengages from the fused portion 107.
Furthermore, according to the evaluation conducted by the inventors, it is found that there is a likelihood that the spark plug 110 may cause firing or ignition failures frequently within the life of the gas engine. When an applied voltage reaches a predetermined level, the spark plug 110 can cause a spark discharge. This voltage level is generally referred to as a required voltage for the spark plug 110. According to the above test result, it is also found that the required voltage for the spark plug 110 possibly exceeds the level of a power voltage (e.g., approximately -35 kV) supplied from apower source to the spark plug 110. It is thus believed that this is a cause of the above-described firing or ignition failures.
Hereinafter, the cause of the above-identified firing or ignition failures will be described in greater detail.
According to the above-described conventional art, a bottom surface 1321a of the hole 1321 provided on the central electrode 103 is not securely connected to a distal end 1511 of the leg portion 151 of the chip 105 by welding. Accordingly, it is believed that the bottom surface 1321a of the hole 1321 is located adjacent to the distal end 1511 of the leg portion 151 with a tiny clearance between them. Otherwise, it is believed that the bottom surface 1321a of the hole 1321 abuts the distal end 1511 of the leg portion 151.
Due to manufacturing accuracies, tiny undulations of several tens .mu.m are generally formed on the confronting surfaces of the central electrode 103 and the chip 105. Thus, even when the bottom surface 1321a of the hole 1321 is brought into contact with the distal end 1511 of the leg portion 151, unavoidable clearance or vacant space (i.e., air layer) of several tens .mu.m exists between the bottom surface 1321a ofthe hole 1321 and the distal end 1511 of the leg portion 151. Therefore, a thermal conductivity is worsened at the boundary between the bottom surface 1321a of the hole 1321 and the distal end 1511 of the leg portion 151.
Accordingly, when the chip 105 receives heat during an operation of the spark plug 110, the heat cannot be effectively released or transferred from the chip 105 to the central electrode 103. The temperature of the chip 105 increases extraordinarily. The chip 105 may be worn out fatally. Thus, a discharge gap (6 in FIG. 2) of the spark plug 110 is increased rapidly. The above-described required voltage is increased correspondingly.